Heating and cooling of a semiconductor wafer in a vacuum environment is a frequent requirement in conjunction with many processes in the manufacture of integrated circuit devices. Because of the ultra stringent requirement for cleanliness during these processes, to avoid introduction of impurities and deposition of particles on the wafer, it has become increasingly essential for advanced integrated circuit manufacture that almost all of these heat transfer operations be carried out in an evacuated chamber. Conventionally, heat transfer mechanisms between a wafer an its environment would involve radiation, conduction at points of contact between the wafer and its support and conduction through the gases between the wafer and the support. Radiation is a very small contributor in the temperature range of concern. Moreover, transfer of heat at vacuum pressures is a complicated phenomenon which is highly dependent on the vacuum pressure employed.
Accordingly, in the prior art, various approaches have been used to increase the heat transfer rate in a vacuum between the wafer and its supporting chuck. Applications of large clamping pressures to increase contact heat conductance has been employed successfully, but it necessarily requires contacting the edge of the wafer. This always results in the formation of particles and of in chipping of the wafer edges. Also, most often, a soft, heat conductive material is employed between the chuck and the wafer to improve the area of actual surface contact. This soft material has tended to stick to the wafer and cause wafer handling and transfer complications.
The most common cooling approach in use in the semiconductor industry at this time is called "backside" cooling and it employs edge clamping of the wafer to a chuck and to apply an increased gas pressure only to the underside of the wafer so that the heating/cooling which gas molecules bounce back and forth between the wafer and the chuck and transfer heat to the colder member. The configuration most generally employs the wafer as a membrane which excludes the somewhat higher pressure heat transfer gases from getting into the lower vacuum pressure evacuated chamber. This structure has been necessary to maintain the cleanliness of the low pressure face of the wafer upon which the processes such as deposition, etch and implantation are carried out.
It was understood in the prior art "backside cooling" that it was beneficial to maintain the gap between the wafer and chuck as small as possible to improve gas molecule frequency of impact. However, the higher back face gas pressure causes the wafer to dome and this causes the gap spacing to have different dimensions across the wafer diameter. To compensate for this problem, the chuck surfaces have been made dome shaped. Also, the backside pressure cannot be much higher than 10 Torr above the vacuum chamber pressure or the wafer will fracture. At the maximum back pressures which have been restricted to approximately 10 Torr or less, the heat transfer rates are highly pressure sensitive. Accordingly, due to local differences in gap spacing between the wafer and the support and the pressure dependency there are spatial variations in the heat transfer rates across the wafer. This has caused a temperature non-uniformity across the wafer.
The Anthony et al., U.S. Pat. No. 3,895,967, discloses an apparatus that bears some superficial similarity to the instant invention. Specifically, backside wafer heating is accomplished in a vacuum chamber by gas conductive heating between the heater plate and the wafer across a large gap. A critical distinction is that Anthony is configured to use a pressure of one (1) atmosphere. At one atmosphere pressure, employing commercially available He, as taught by Anthony, the concentration of the impurities introduced by the He are so high as to make the process impractical because the impurities diffuse into the wafer. Anthony employs the higher pressure regime for the improved heat transfer rate. In this one atmosphere pressure range, since the gas heat transfer is insensitive to pressure variations, the effect of the large gap between the wafer and the plate as used by Anthony is acceptable.